Generally, in rechargeable batteries such as lithium ion battery and lithium polymer battery, an electrolyte and an electrode assembly are contained together in a can. The electrode assembly includes an anode electrode, a separator, and a cathode electrode. The anode electrode is formed with a foil shaped anode current collector and an anode active material coated on a surface thereof, while the cathode electrode also is formed with a foil shaped cathode current collector and a cathode active material coated on a surface thereof. The current collectors of the electrodes have non-coated portions on which the active material is not coated and to which a conductive tap is usually welded in order to connect the electrode to an exterior circuit.
In order to increase the capacity of the rechargeable battery, the electrode assembly is adapted to have a stronger tension during the winding of the electrode assembly. The stronger the tension is, the larger the area of the electrode assembly which can be wound within the same volume becomes.
However, various problems may occur when the tension of the electrode assembly increases. One of the problems is that the end portion of the active material layer having one polarity, i.e. a sharp end portion of the active material forming a boundary between the non-coated portion of the current collector and the active material, can extend through a separator so as to cause an electric short circuit in an active material layer or on an electrode having the other polarity. The electric short circuiting of the active material or the electrode causes the deterioration of the stability and the reliability of the rechargeable battery.
It is known that the above-mentioned problem frequently occurs in the electrode assembly wound in a cylindrical shape. Specifically, it is believed that the end portion of the active material layer applies stress to any portion on a peripheral surface of a can because the can also has a cylindrical shape in the cylindrical rechargeable battery.
Further, in the rechargeable battery, it has become known that the cathode active material expands somewhat in an initial charging and discharging of the rechargeable battery. Therefore, when the active material expands in the electrode assembly wound with the increased tension as described above, the sharp end portion of the active material layer may extend through the separator so as to cause an electric short circuit in the active material or on the electrode having the opposing polarity.
Moreover, the temperature of the rechargeable battery generally rises during the discharging of the rechargeable battery. Thus, the rigidity of the separator may decrease to allow the end portion of the active material layer to easily cut the separator, thereby causing the electric short circuiting more easily.